(a) Field of the invention:
The present invention relates to an infrared detector for detecting moving direction of human body, etc. by detecting infrared rays emitted therefrom.
(b) Description of the prior art:
It is widely practiced to detect invaders or presence of persons by using pyroelectric infrared detectors, most of which are composed, as shown in FIG. 1 of pyroelectric elements 1 and 2 having different orientations of polarization and connected in series or parallel (connected in series in FIG. 1) so as to oppose the orientations of polarization to each other. This formation is adopted for cancelling noise since the pyroelectric material used as the pyroelectric elements 1 and 2 generally has a piezoelectric property and produces noise due to the piezo effect when vibration is applied to the detector. The formation is adopted also for cancelling noise which is produced by a cause other than the infrared rays entering through the window when ambient temperature varies abruptly. In such a detector, the light receiving electrodes of the pyroelectric elements 1 and 2 have a rectangular shape. In practice, the detector is so designed as to focus the infrared rays emitted from a detected object onto only one of the electrodes by a condenser consisting of lens, mirror, etc. Accordingly, when an object moves so as to cross the two electrodes, the two electrodes are irradiated alternately by the condensed rays and cannot be irradiated simultaneously. In other words, one of the two electrodes always performs the role of the compensating electrode. In addition, the reference numeral 3 represents an FET for impedance conversion whose gate G is connected to the negative electrode of the pyroelectric element 1, the reference numeral 4 designates a leak resistor connected between the gate G of the FET 3 and the negative electrode of the pyroelectric element 2, and serving to prevent the gate of the FET 3 from being saturated when an excessive input is applied, and the reference numerals 5 and 6 denote output terminals.
As for applicational purposes of the conventional infrared detectors, they are used mainly for detecting invasion or presence of bodies emitting infrared rays. In the recent years where the detectors of this type widely prevails, it is demanded to use the infrared detectors in more sophisticated ways. For example, it is demanded to use the infrared detectors for detecting not only invasion but also invading direction. Concretely, it is demanded to announce optimum messages while detecting passing directions of customers, or start illumination and ventilation fan while detecting entrance and exit into and out of a toilet.
In order to meet the demand described above, two methods have hitherto been contrived for detecting moving directions with the conventional infrared detectors.
One of the methods is to use two infrared detectors of the conventional type described above. By arranging two detectors 7 and 8 and connecting signal processing systems 9 and 10 to the detectors respectively as shown in FIG. 2, obtained signals are compared by a discriminating circuit 11 to discriminate which of the signals is earlier for operating a display circuit 12. In this case, it is necessary to arrange an optical member (usually a plate-like member) for shielding the infrared ray between the detectors 7 and 8. This member is required for allowing the infrared ray emitted from an invader to be incident always first on the detector located nearer the invader and then on the detector located farther from the invader. The detecting system of this type has a simple formation and can function almost without fail. In this case, however, two detectors (each comprising a detecting pyroelectric element and a compensating pyroelectric element) are necessary, thereby producing a defect that number of required parts is increased and the detecting system occupies rather large space or has large dimensions.
The other of the methods is to detect passing direction by discriminating phase of output signals from a single detector. The discrimination mode will be described with reference to FIG. 3. When an object to be detected invades into the detection area while moving in the direction A before an infrared detector having the internal circuit shown in FIG. 1, the output circuit 6 provides an output signal having the waveform shown in FIG. 3A. When an object to be detected invades into the detection area while moving in the direction B, in contrast, the output signal has the waveform shown in FIG. 3B. Comparison of these waveforms clarifies that the positive and negative side are inverted to each other on these waveforms. This fact indicates a possibility to discriminate the moving direction by detecting the phase of the output signal. If such discrimination is possible, it seems that a direction detector can be composed only of a single detector, an amplifier and a phase discriminating circuit. However, an actual prototype of this direction detector reveals a great defect thereof. Speaking concretely, the output signal waveform is inverted even in the same moving direction when background temperature is varied from a level higher than surface temperature of an object to be detected to another level lower than the surface temperature by a seasonal cause, etc. As a result, the discrimination result obtained in summer is reverse to that obtained in winter when the detector is used for detecting moving direction of a human body. Further, due to the fact that the output signal waveform is irregular as shown in FIG. 3 unlike the regular sine waveform, the direction detector has another defect that it can hardly discriminate the phase accurately, and is therefore low in reliability and practical utility.